Electric machines, such as synchronous machines, can be operated in generator operation or in motor operation, together with active power converter circuits, fashioned for example as a bridge circuit. Such a bridge circuit is shown schematically in FIG. 1, explained below. Such power converters standardly have a number of half-bridges each having two controllable switching elements, and can be used for the rectification of an alternating or three-phase current produced by a generator, and/or for inversion during the commutation of an electric motor. A so-called freewheeling diode is standardly provided parallel to each switching element. The blocking direction of the freewheeling diode corresponds to the technical direction of current between the direct voltage terminals. The named switching elements can for example be fashioned as metal oxide semiconductor field-effect transistors (MOSFET). In a corresponding MOSFET, the freewheeling diode is realized as an inverse diode.
Both in rectification operation and in inversion operation (for motoric operation of the electric machine), the switching elements of a power converter half-bridge are each controlled in alternating fashion, i.e. are switched into a conductive state. Here the switching elements react with a certain switching delay that is a function of many influencing factors, such as parasitic inductances, component tolerances, and the actual level of the current at the direct voltage terminal.
During switching, a so-called “hot path,” i.e. a simultaneous conductive state of both the switching elements, should be avoided in order to prevent a short circuit. In order to take into account the switching delays during operation, a so-called dead time can be provided between the control phases for the switching elements of a power converter half-bridge. This is a pause during which neither of the two switching elements of a power converter half-bridge are controlled. The switching elements of a power converter half-bridge are thus controlled in alternating fashion, each interrupted by controlling pauses.
The dead time must be chosen to be long enough to ensure that no one switching element of the half-bridge has opened before the other switching element of the half-bridge closes. On the other hand, the dead time must be kept as short as possible in order to avoid losses at the freewheeling diodes that occur as long as both switching elements are open. Therefore, the present invention seeks to improve the setting of the dead time in the controlling of power converters, in particular in inversion operation, and to enable more reliable operation.